1. Field of the Invention
The present invention relates to a magnetic disc system. More particularly, it relates to a track access control system for a magnetic disc system which uses an offset obtained from servo-tracks provided in a magnetic disc, in addition to data tracks, as a correction value of a track access, and enables a host computer to access data on physical tracks in the magnetic disc by using logical tracks.
2. Description of the Related Arts
Recently, in rotation type magnetic disc systems, a distance between tracks on a magnetic disc has become short due to requirements for a high density and a large capacity of data storage. In those magnetic disc systems, the displacement of the tracks, i.e., an offset, caused by an error in magnetic head installation, temperature changes, etc., cannot be neglected, and a correction of this displacement is necessary for data access.
Accordingly, magnetic disc systems in which servo tracks storing servo positioning information are provided are known. In those magnetic systems, the magnetic head is positioned at the servo track to read the servo positioning information, a displacement of the servo track is detected, and the detected displacement is used as an offset value for correcting the displacement and positioning the magnetic head at the subsequent head accessing when access to a data track is required.
In a magnetic disc system, two types of track address, i.e., a physical track address and a logical track address, are defined. The physical track address designates actual tracks comprised by data tracks and servo tracks on the magnetic disc, but the logical track address designates only data tracks. A plurality of magnetic discs may be provided in the magnetic disc system, but in order to save effort storing servo information into all magnetic discs, the servo tracks may be partially formed on at least one face of a magnetic disc. Accordingly, the face of the magnetic disc in question is provided with the servo tracks and the data tracks. In this system, the physical track addresses designating the servo track addresses and the data track addresses differ from the logical track addresses. Another face of the magnetic disc and the other magnetic discs are provided with only data tracks.
In this case, the physical track addresses coincide with the logical track addresses.
As described above, handling of the track address is relatively difficult, and when the handling is to be processed by a host computer, the programming of the host computer is complex (JPP49-117007).
Another prior art magnetic disc system disclosed in U.S. Pat. No. 4,396,959 also suffers from the following disadvantages:
a) since two or more servo sectors cannot be provided in each track, i.e., only one servo sector is provided in each track, if a defect occurs in the servo sector, the offset measurement therefor cannot be effected, and PA1 b) due to the provision of the servo sector in each track a rotation speed of the disc must be reduced and, the density of the memory storage in a circumferential direction must be increased.